1. Field of the Invention
This invention relates to an epitaxial wafer for GaP (gallium phosphide) pure green light-emitting diode and to a GaP pure green light-emitting diode fabricated using the epitaxial wafer.
2. Description of the Prior Art
GaP has a relatively large band gap of around 2.26 eV, corresponding to the green wavelength, and is therefore used as a material for greenish light-emitting diodes. Since GaP is an indirect transition semiconductor, its light-emitting efficiency is ordinarily increased by doping the n-type GaP epitaxial layer forming the p-n junction with nitrogen, which functions as light-emitting centers.
As the light emitted by this diode has a wavelength of about 565-570 nm, however, its color is yellowish-green.
Unlike this yellowish-green light-emitting diode, another known light-emitting diode achieves pure green light emission in the 555-560 nm wavelength range by direct recombination of electrons and holes, without doping the n-type GaP epitaxial layer forming the p-n junction and constituting the light-emitting layer with nitrogen or other impurity to function as light-emitting centers. The general structure of the epitaxial wafer for this GaP pure green light-emitting diode is shown in FIG. 1. Reference numeral 1 in FIG. 1 designates an n-type single crystal GaP substrate, 2 a first n-type GaP epitaxial layer, 3 a second n-type GaP epitaxial layer not doped with nitrogen, and 4 a p-type GaP epitaxial layer. Reference numeral 5 designates a substrate constituted of the n-type single crystal GaP substrate 1 and the first n-type GaP epitaxial layer 2 formed thereon.
This epitaxial wafer for a pure green light-emitting diode is generally fabricated by impurity overcompensation liquid-phase epitaxial growth. An example of this fabrication method utilizing the well-known sliding boat method is briefly explained in the following.
The substrate 5 ordinarily used consists of the n-type single crystal GaP substrate 1 and the n-type GaP epitaxial layer (first n-type GaP epitaxial layer) 2 formed thereon as a buffer layer.
The substrate 5 formed with the first n-type GaP epitaxial layer 2 (buffer layer) is set in the substrate holder of the boat, and the Ga metal and GaP polycrystal for forming the epitaxial growth melt are placed in the melt container. The slide boat is inserted into an epitaxial growth furnace with the substrate and epitaxial growth melt still separated. The furnace is heated to a prescribed level of around 1000.degree. C. in a stream of hydrogen to melt the GaP polycrystal into the Ga metal to the point of saturation. At this time, sulfur is added to the epitaxial growth melt by, for example, simultaneously passing a prescribed amount of H.sub.2 S gas together with the hydrogen stream. Other methods of sulfur addition are also available, including that of adding sulfur or a sulfur compound directly to the epitaxial growth melt.
After the epitaxial growth melt has stood long enough to become uniform, the substrate holder of the slide boat and the melt container are slid to guide the epitaxial growth melt onto the substrate. The melt and the substrate are then gradually cooled to about 900.degree. C. to grow the second n-type GaP epitaxial layer 3. The second n-type GaP epitaxial layer 3 includes sulfur as a dopant but is not doped with nitrogen.
With the temperature maintained, zinc is supplied to the epitaxial growth melt from a gas phase, for instance, in an amount sufficient to impart the epitaxial growth melt with p-type conductivity by compensating the sulfur therein.
Gradual cooling to a temperature of about 800.degree. C. is then effected to grow the p-type GaP epitaxial layer 4.
When the growth is complete, the substrate and the epitaxial growth melt are separated and cooled to normal room temperature. A light-emitting diode fabricated from the epitaxial wafer obtained in this way emits pure green light with a peak wavelength around 555-560 nm.
In this GaP pure green light-emitting diode, either the p-type layer or the n-type layer can be made the light-emitting layer by appropriately selecting the carrier concentration of the p-type layer and the n-type layer in the p-n junction section. Specifically, the n-type layer becomes the light-emitting layer when the carrier concentration on the side of the p-type layer interfacing with the n-type layer is made higher than the carrier concentration on the side of the n-type layer interfacing with the p-type layer and the p-type layer becomes the light-emitting layer when the carrier concentration on the side of the n-type layer interfacing with the p-type layer is made higher than the carrier concentration on the side of the p-type layer interfacing with the n-type layer.
The purity of the light emitted by the aforesaid pure green light emitting diode is superior to that emitted by the yellowish green light-emitting diode and the light color is more appealing. Since it does not use nitrogen as light emission centers, however, it is lower in brightness than the yellowish green light-emitting diode. Higher brightness is therefore required to melt the demands of the market.
Methods for increasing the brightness of a pure green light emitting diode include, for example, that taught by Japanese Patent Public Disclosure 59-22376 (hereinafter JP-A 59-22376). In this method, brightness is heightened by using the impurity compensation method with reducing the donor concentration in the p-type Gap. However, the increased brightness afforded by this and other prior-art methods is not sufficient to satisfy increasingly severe user requirements.
One object of this invention is to provide an epitaxial wafer for fabricating GaP pure green light emitting diodes with greatly enhanced emission brightness wherein an n-type epitaxial layer forming a p-n junction to function as a light-emitting layer is not doped with impurity functioning as light-emitting centers such as nitrogen.
Another object of the invention is to provide a GaP pure green light-emitting diode fabricated using the epitaxial wafer.